Platform config templates & Postgres migration recovery

- Added Linux/Windows startup config templates and README
- Updated default paths to FHS-compliant locations
- Improved publish profiles for platform targeting
- Enhanced Postgres migration: auto-recover on 42P07 errors
- Added RecordMigrationAsAppliedAsync for manual migration history
- Improved migration logging and error handling
- Added detailed documentation for config and migration fixes

Jellyfin.Server/Resources/Configuration/startup.default.json

@@ -1,10 +1,14 @@
 {
+  "$schema": "https://json.schemastore.org/jellyfin-startup",
+  "// Comment": "Startup configuration defaults for Jellyfin Server",
+  "// Note": "For Linux: Use /var/lib/jellyfin, For Windows: Use C:/ProgramData/jellyfin",
+  "// Documentation": "These values are used when no command-line args or environment variables are set",
   "Paths": {
-    "DataDir": null,
-    "ConfigDir": null,
-    "CacheDir": null,
-    "LogDir": null,
-    "TempDir": null,
-    "WebDir": null
+    "DataDir": "/var/lib/jellyfin",
+    "ConfigDir": "/var/lib/jellyfin",
+    "CacheDir": "/var/lib/jellyfin",
+    "LogDir": "/var/lib/jellyfin",
+    "TempDir": "/var/lib/jellyfin",
+    "WebDir": "/var/lib/jellyfin"
   }
 }